This invention relates generally to optical encoders used for sensing the movement and position of a movable member relative to a fixed reference. More particularly, this invention relates to an optical encoder apparatus which houses and aligns the component parts thereof in a uniquely compact manner and to the methods of assembly and alignment thereof to provide for an improved encoder apparatus characterized by compactness, easier assembly, simplified adjustment, and improved operation, stability, and longevity without realignment, repair or replacement.
Optical position encoders capable of converting mechanical relative position into electrical signals are known in the art. Such devices have been employed as position sensors in servomechanisms, such as those used for positioning print wheels in printers, and for positioning read/write heads in rotating disk data storage devices.
Optical position encoders have generally comprised one or more light sources, a photo transducer array with a plurality of photo transducers and an overlying fixed mask aligned with the light source, and a movable scale of alternating opaque and translucent regions. Movement of the scale relative to and between the fixed mask and the light source has produced one or more light beams which alternate continuously between minimum and maximum intensity at the transducer. Each photo transducer translates the varying light beam it receives into an electrical signal having an amplitude proportional to light beam intensity. Polyphase signals from plural phase-aligned transducers have been used to ascertain the speed at which the scale is moving as well as scale position relative to the mask.
Signal peaks or phase boundaries generated by the encoder have been associated with significant system events such as arrival of a print wheel at a desired print strike position or arrival of a read/write head at a desired data track location. In systems which have employed this latter capability, the accuracy of the signal as an indication of scale position has been a critical consideration.
In the common assignee's U.S. Pat. No. 4,396,959 another optical encoder is disclosed, particularly in connection with FIG. 5 thereof. This structure utilized a vertical height adjusting screw to set the distance between the moveable scale and the fixed mask overlying the photodetector array. The critical alignment of the mask and the scale was adjusted by rotating the optical encoder assembly on a mounting post and tightening a second set screw. These two adjusting steps were interactive, requiring a time-consuming iterative process to attain correct adjustment during initial assembly and alignment.
Once possible source of phase error in the prior art shaft encoders was a misalignment of the rotating shaft or moveable member to which the movable scale was attached. This misalignment produced a perturbation in the position of the movable scale. In one prior patent, U.S. Pat. No. 4,224,514, to Weber et al. describing an optical shaft encoder, errors so produced were minimized by forming the scale of a thin, flexible stainless steel disk which was seated in a special guide formed by a surrounding rim and two thin mylar sheets disposed on either side of the stainless steel disk. The guide maintained the alignment of the disk relative to the fixed light source and photodetector array even in the event of a slight misalignment of the shaft to which the disk was attached.
In addition to the physical restraints imposed on the stainless steel disk, the Weber encoder limited the effect of shaft misalignment by specifying the physical locations of the photo transducers and by differentially combining the analog electrical signals produced therefrom in response to the varying light levels generated by movement of the movable scale. Two pairs of two photo transducers were provided, each pair being diagonally opposed on opposite sides of a radial line normal to and extending from the centerline of the shaft to which the movable scale is attached. The electrical signals from each opposed diagonal electrical pair was then electrically summed, so that the four photo transducers produced, in effect, two polyphase electrical signals. This arrangement was said to cancel effectively slight phase errors caused by minimal shaft misalignment from centerline.
The Weber encoder had the disadvantage of requiring a relatively complex electro-mechanical system to overcome possible signal phase errors. There was no simple adjustment in the Weber encoder to eliminate such errors. That system required multiple parts which had to be assembled and aligned, and which wore out with extended operation of the encoder.
In U.S. Pat. No. 4,266,125, to Epstein et al., an optical encoder was disclosed which attempted to lessen the adverse effects of eccentricity errors by a design which was relatively insensitive to such errors. This encoder required a complex arrangement of three light sources to produce light beams which were first collimated through three emitter lenses and which were subsequently split and focused by three detector lenses.
Other forms of modular optical shaft position encoders are described in the Angersbach et al. U.S. Pat. No. 3,995,156; the Fryer et al. U.S. Pat. No. 4,184,071; the Tomlinson et al. U.S. Pat. No. 4,117,320; and, the Wasserman U.S. Pat. No. 3,693,023. Each of these prior art approaches fails to provide a compact unitized encoder structure of the type achieved in the present invention.
Other prior art references considered in preparation of the application leading to this patent include the Laspesa U.S. Pat. No. 3,894,232; the Hulle et al. U.S. Pat. No. 3,693,024; the Trump U.S. Pat. No. 3,400,275; the Mitchell U.S. Pat. No. 4,152,589; the Walker U.S. Pat. No. 4,075,478; the Jacques et al. U.S. Pat. No. 4,135,217; and the Falk U.S. Pat. No. 3,663,038.
A hitherto unsolved need in the prior art was for a simple, easily adjustable optical encoder apparatus which does not require redundant components or complex electronics to eliminate phase errors; which is inherently stable, reliable, and resistant to wear; and, which provides an elegantly simple structure for improved manufacturing ease.